this man's job when he was at the secret service -- protect president obama with 1.8 million people on the washington mall gathered for his first inauguration. >> you can't guarantee anything. all you do in the field of security is reduce the probe ability of an event like this to something so miniscule if something does happen your response element afterward is enough to keep the casualties to a minimum. >> many americans have grown tired of war, tsa checkpoints and heavy security at sporting events. former national director of counterterrorism says the threat remains very real. >> if this turns out to be either domestic or international terrorism it will remind people of the difficulty of the tasks that we have been working against over the past ten years. >> reporter: experts say that while we can certainly prevent and address more security issues than we did before 9/11, trying to protect a soft target like a marathon is just increasingly difficult and they point out that a terrorist only has to find the weakest, softest point and attack there. of course that's where we're all most